A frequent problem in the case of structures and objects submerged in moving bodies of water is the local scour or erosion of material of the bottom in which they lean on, induced by the alteration of the current or flow caused by the presence of those structures and objects. This alteration consists of local increases of the velocity and in the onset of secondary flows and of turbulent wakes with whirls or vortices. The local scour removes material from the bottom around the structures and objects and can put at risk the support of the same, threatening their stability and security and those of the corresponding superstructure, if any.
In spite of the technical development achieved through research (theoretical, experimental and with the help of numeric simulation and with field studies), carried out in countless institutions in many countries, and in spite of the interest of the official institutions in charge of the ground communications infrastructure, and in spite of the enormous amounts of money invested in the search of solutions, scour is the main cause of the collapses of bridges, worldwide. 60% of those disasters are due to this phenomenon; because of that, nowadays it is still a factor of paramount importance in the design of these structures and an urgent technical challenge to solve (Refs. 1, 2, 3, 4, 5, 6, 30). (See LIST OF PRIOR ART REFERENCES, at the end of BACKGROUND ART).
In the state of Texas (United States), that has some 48,000 bridges and where between 200 and 300 bridges a year are built at an average cost of 500,000 dollars each, 1,000 bridges collapsed between 1961 and 1991; in the United States, 18,000 bridges are considered in critical state with regard to scour (Ref. 5). The great number of bridges (more than 575,000 in the United States, more than 156,000 in the United Kingdom) (Refs. 7, 8, 9) gives an indication of the problem dimensions and its dramatic economic impact. The costs directly related with the bridge collapses are always very high. For example, 19% of the United States federal emergency funds used in the item of highways is expended in the restoration of bridges; in the period 1980-1990, it amounted to an average of 20 million dollars annually (Ref. 3). It should be added to the previous costs the indirect costs due to the serious affectations to the roads and the dysfunctions in many activities; such costs can be even higher than the direct ones: the Federal Highway Administration of the United States estimates that these indirect costs can be five times the direct ones (Ref. 30). There is also a cost of prevention of those disasters: in the United States, about 15 million dollars have been spent in the last 8 years in research on bridge collapses, mainly due to scour in sandy bottoms (Ref. 5).
The collapses of bridges also imply an important risk for the public security: there have been lost human lives in those disasters.
The collapse of a bridge due to scour generally begins with the loss of support of one or more piers, which are the intermediate columns that support the superstructure of the bridge. One of the abutments can also fail; they are the supports at the ends of the bridge, where this structure rests on the riverbanks.
Other examples of structures susceptible to damage due to local scour are piles, columns, supports of structures or equipment or machines, pipelines and other conduits and similar structures, leaning, anchored, sunk or buried in the bed or bottom or in the riverbanks of a ravine, a body of water or an artificial channel or in a fluvial, lacustrine, estuarine, coastal or marine environment.
Local scour is produced by a complex turbulent flow that is mainly the effect of two independent, well-known mechanisms studied by multiple researchers. A brief explanation of those two different causes of the phenomenon follows which belongs to the state of the art and constitutes the base of my invention.
a). First mechanism: The horseshoe vortex. FIG. 1 shows a cylindrical submerged structure or object 10, leaning on a scourable or erodible bottom 11. The flow 12 that impinges against the border or attack area of the structure (the area that directly faces the current or flow) is deflected downward, generating this way a secondary flow that produces the so-called horseshoe vortex 13 when colliding against the bottom 11. This vortex surrounds the structure or submerged object and spreads downstream, removing material from the bottom 11 around the structure; this material is then transported by the current, giving place to the scour hole 15.
This first mechanism is very important in the case of submerged structures whose position is vertical or near it.
b). Second mechanism: von Karman vortices (FIG. 1).—The flow that surrounds the submerged structure or object 10 produces the vortices 14, called von Karman vortices. These vortices appear periodically and alternately from one side and the other of the structure and are carried away by the flow. These vortices, as small tornados, remove particles from the bottom 11 and put them into movement; the flow transports them and this is the second scour mechanism. The von Karman vortices are an important part of the wake caused by the presence of the structure or object in the flow.
The intensity of this second mechanism is related with the behavior of the boundary layer, the fluid layer of small thickness that flows in contact with the submerged structure or object. This boundary layer moves while remaining in contact with the structure until separation of flow occurs: the boundary layer comes off the structure and is carried away by the flow. FIGS. 2 and 3 show a well-known phenomenon of Fluid Mechanics: the intensity of the von Karman vortices and the dimensions of the turbulent wake depend, the remaining flow characteristics being equal, on the location of the points 16 of the perimeter of the structure 10 where the separation of flow occurs; the farther toward downstream are those points 16, the smaller dimensions the turbulent wake 17 will have and the less strong the von Karman vortices will be. In FIG. 3, the points of separation of flow 16 are farther downstream than in FIG. 2, so the produced turbulent wake 17 is smaller and the vortices have less intensity and, therefore, they produce less scour.
My invention acts against both scour mechanisms, as will be seen later in this document.
If the depth of the scour hole 15, basically due to the action of the two described mechanisms, surpasses certain magnitude, the support of the structure is reduced and its security is at risk.
In the case of structures and objects totally buried in the bottom or bed (such as river-crossing pipelines and submarine ducts), the scour, if it uncovers them, becomes more intense because of the vortices induced by the same structures and objects and exposes these to damages; this is why they may require frequent maintenance, and sometimes, repair or reconstruction.
As a conclusion, in all cases of structures and objects submerged in a moving liquid environment and leaning on or buried in the bottom, it is convenient to reduce the vorticity causing the scour, to increase the security of the structures or objects, to prolong their work life and to reduce the maintenance or repair costs.
The current state of the art consists on facing the problem with three main types of measures intended to reduce the scour effects around the submerged structure or object:
1. The protection of the bed or bottom near the submerged structure or object, using one or more of the following resources: rock, monolithic precast and cast-in-place concrete structures, crushed concrete, armor stone with granular filters (riprap), mattresses or mats of several types constituted by such heavy elements as bags made of plastic meshes or geotextiles containing concrete or stones, metallic cages containing stones (gabions), concrete blocks tied to each other by steel cables, buried columns of waste tires united by metallic elements, injections of fluid cement in the bottom under and around a submerged structure and mixing of this cement with the bottom granular material with the help of machines to solidify the support area, the generation of upward currents by means of small hydraulic machines to counteract the descending secondary flow, and other resources (Refs. 4, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27).
2. The construction on or near the submerged structure or object of some elements (generally, of reinforced concrete), whose position and shape help divert the flow and the scour away from the structure or submerged object, such as triangular or semicircular endnoses, “V” shaped flow deflectors, protection slabs, collars, sacrifice piles and other components (Refs. 4, 9, 10, 28, 29).
3. The construction or setting of the structure foundation to depths greater than the scour depths estimated by means of the available calculation formulae. The reason of this measure is that the formulae don't give reliable results; their error margins are generally big. The usefulness of the formulae is also limited because they don't consider the cases of complex flows, like those that include surf or currents; also, they don't take into account complex geotechnical characteristics of the bottom and are only applicable to simple shapes of piers.
The application of these three types of measures is always expensive: it requires additional materials, time and work, it implies the use of elements and materials whose preparation, transport and placement require special personnel, heavy equipment and particular techniques. The presence of surf or high-velocity flow complicates the maneuvers. Also, this type of solutions generally requires maintenance, which increases costs.
From the functional point of view, the described measures, which constitute the main body of the current state of the art, have as objective to reinforce the bed or bottom or to move the phenomenon of scour away from the structure or object to be protected, or set the structure to a depth that responds more to a collapse fear than to a rational, engineering decision. That is, the state of the art attempts, at great cost and without much success, to reduce the effects of local scour, without attacking to the cause of the phenomenon itself. The statistics of vulnerability of the submerged structures to the effects of local scour show that the protection obtained nowadays is faulty and that there is an urgent need of a better solution.
In the case of structures and objects buried in the bottom of a liquid mass in which there are currents, such as pipelines and conduits that cross water courses or lacustrine, estuarine, coastal or marine areas, the state of the art recommends measures like adding anchors or fastenings to the bottom and setting of the structures to considerable depths, which represent big costs.
The current state of the art doesn't consider, in none of the mentioned cases, the control of the hydrodynamics, responsible of the two main mechanisms that produce scour that were already described.
Regarding the drag force exerted by the moving water on the submerged structures and objects, or due to the movement of the objects in a motionless aquatic medium, in many instances it is convenient to diminish it to reduce the stresses those structures and objects must endure, due to security, economy and operative reasons.
The state of the art in respect to the drag force includes applying to the structures susceptible to the drag force some of the following measures. For fixed structures: giving the structures a massive nature, to increase their inertia, and additional structural reinforcement. For pipelines and other ducts: strong anchorages to the bottom and a structural design revised to resist the drag force. For semifixed structures: strong anchorages to the bottom and, if needed, special positioning mechanisms to counteract the movement and displacements. In the case of vessels: an appropriate hydrodynamic profile design.
The mentioned measures are, in general, costly.
In this document, in the description of the boundary layer behavior, it is stressed that the farther downstream the flow separation points are located, the lower will be the intensity of the turbulent wake. A weak turbulent wake produces less alteration in the flow, and the latter loses less energy and so the drag force exerted on the submerged structure will be smaller; this reduction occurs also if the structure or object is moving and the water is at rest.
The drag force reduction by diminishing the intensity of the turbulent wake is a known principle of Fluid Mechanics; however, it doesn't belong to the state of the art in the field of my invention.
Note: this invention has the same technical basis as the one I have named MOLDE PARA CREAR RUGOSIDAD ARTIFICIAL CONTRA LA SOCAVACIÓN, with applicant file reference 12136MOL, whose application I present simultaneously with this one and which refers to a different device.